Locking units described above are used in particular with the parking brakes in automatic transmissions.
A vehicle with manual gear shifting can be secured against unintended movement during standstill when the engine has been turned off by engaging a gear. With automatic transmissions, however, this is not possible because there is no frictional connection with the driven wheels when the engine has been turned off. Therefore automatic transmissions have a mechanic locking of the transmission driven shaft which has to be activated in the park position, that means it has to be set, and it prevents the car from moving.
The locking unit there acts together with the hydraulic circle of the automatic transmission. When the parking brake is set it is usually provided that the piston is unlocked. In the drive position, that is when the parking brake is not set, the piston impinged with pressure is locked. Usually there the piston rod of the piston acts on a catch which engages or does not engage according to the position of the parking brake into the driving lane.
The locking units used for that have balls as operating elements which are movably supported radial to the armature bar and because of the movements of the armature bar during driving are pressed outward in order to engage with a catching element and thus lock the movement of the piston.
In the currentless condition (usually in the park position) the unlocking position of the piston is achieved by means of a spring which moves the armature bar against the otherwise prevailing holding force of the electromagnet in such a way that the balls can recede radial to the inside and thus the movement of the piston is no longer blocked. The arrangement there is such that the unlocking position (park position) is passively safe, that means in the currentless condition the movement of the piston is not blocked, the piston is unlocked.
In the example shown here according to the state of the art the meshed park position corresponds with the unlocked piston, the out-of-gear park position corresponds with the locked piston. In the sense of the invention, however, it is also possible to reverse this connection, that means to use an unlocked piston when the parking brake is out-of-gear and vice versa.
The use of the balls as operating elements, however, leads to some disadvantages. The balls run in oil which, for example, is in contact on the pressure side on the piston. Therefore it cannot be excluded completely that, for example, rubbed-of parts of the gear or other soiling (for example chips) are being deposited into the locking region and thus obstruct the movement of the balls. If, however, the movement of the balls is not secured the function of the locking unit altogether is not guaranteed. Therefore the arrangement according to the state of the art is susceptible to soiling.
Furthermore a ball has only point-to-point contact to its surrounding surfaces. That means that the locking forces act only on small surfaces and the corresponding pressure stress is enormous. In the state of the art this is compensated by accordingly especially hardened parts, however, these parts are correspondingly expensive to manufacture.